The present invention relates to the regeneration of solid catalysts. More specifically, the present invention relates to an apparatus for the regeneration of fluidized solid catalyst used in the catalytic cracking of hydrocarbons.
In usual hydrocarbon cracking processes, it is customary to contact a hydrocarbon feed stock with a fluidized solid catalyst in a reaction zone to effect at least partial conversion of the hydrocarbon to certain, desired conversion products. In the course of the reaction, there is a concomitant deposition of coke on the solid catalyst particles. To maintain efficiency of the reaction, the spent catalyst having the coke deposition thereon is continuously stripped and passed, as a dense phase, into the dense phase of a regenerator wherein the spent catalyst is contacted with an oxygen containing gas, e.g. air, thereby effecting combustion of at least a portion of the coke and regeneration of the catalyst. The regenerated catalyst is continuously withdrawn from the regenerator and returned to the reaction zone.
U.S. Pat. No. 3,394,076 discloses a process and apparatus for the regeneration of such fluidized solid catalyst. In the patented method and apparatus, a swirling motion is induced in the dense phase bed of the regeneration zone to extend the path of the solid particles traversing the regeneration zone and hence increase residence time of the catalyst particles in the regenerator. As noted in the subject patent, the extended path of catalyst particles travel is achieved by tangentially introducing the spent catalyst as a dense phase into the regeneration zone and withdrawing regenerated catalyst from the regeneration zone at a point circumferentially remote from the point of tangential introduction. The swirling motion imparted to the particles is further enhanced by passing effluent gases and entrained solids leaving the dense phase bed of the regeneration zone to gas-solid separating means oriented to receive the effluent gas traveling in the direction of the swirling motion without any substantial reversal of direction. Solids separated in the gas-solid separating zone are preferably returned to the dense phase bed of the regeneration zone in the direction of the swirling motion. Additionally, the oxygen containing regeneration gas may also be introduced into the dense phase bed of the regeneration zone in the direction of the swirling motion.
The above described method of regenerating spent catalyst employs apparatus including a generally cylindrical drawoff standpipe in open communication at its top with the interior of the regeneration chamber at a point above the operating level of the fluidized bed therein, the standpipe containing at least one aperture or window in its lower portion in open communication with the interior of the regeneration chamber at a point below the operating level of the fludized bed therein. A baffle is affixed to the standpipe below the aperture which baffle is inclined upwardly and outwardly so as to form a scoop. Additionally, there is a conical section depending from the cylindrical section of the standpipe and extending to the bottom of the regenerator. While the apparatus described above provides an efficient means for regenerating spent catalyst, it suffers from several disadvantages. For one, the rather complex configuration of the standpipe makes fabrication somewhat difficult. Additionally, because of its rather substantial length it is necessary that the standpipe be supported internally of the regenerator to give it sufficient rigidity. The provision of support members may interfere with the desired swirling motion of the catalyst particles and hence make regeneration less efficient. Additionally, the "windows" or apertures in the standpipe reduce the structural integrity of the standpipe and hence make it subject to failure. For example, regenerators employing the above described standpipes as catalyst drawoffs are subject to collapse at the point where the windows or apertures are cut with the result that the latter are virtually closed and preclude catalyst removal. When this occurs, it is necessary to shut down the regenerator and replace the catalyst drawoff standpipes.